The present invention relates to a method for producing a sulfonimide or its salt. Sulfonimide compounds are useful as Lewis acid catalysts and ion transport agents, in the fields of organic compound syntheses, electrolytes and the like.
U.S. Pat. No. 5,723,664, corresponding to Japanese Patent Publication JP-A-8-81436, discloses a method for producing a sulfonimide or its salt. The product obtained by this method, however, may contain an impurity such as (1) a sulfonamide represented by the general formula RfSO.sub.2 NH.sub.2 or (2) a sulfonic acid represented by the general formula RfSO.sub.3 H. The removal of this impurity may prevent a problem of the decrease of potential of an aluminum collector and other problems, when the product (i.e., lithium salt of sulfonimide) is used for electrolyte of a lithium cell.